The tuberculin skin test is based on the fact that mycobacterial infection produces delayed hypersensitivity to certain products of the organisms contained in culture extracts called tuberculins. The intracutaneous injection of tuberculin in sensitized individuals results in an area of induration, with or without surrounding erythema, which varies in size and intensity according to the dose of tuberculin and the sensitivity of the individual. This cell-mediated or delayed hypersensitivity reaction is characterized by its delayed course, reaching a peak at 48 to 72 hours, and by induration, due largely to cellular infiltration at the site of the antigen injection in sensitized persons called "reactors". Occasionally vesiculation and necrosis may occur. Not all reactors are infected with M. tuberculosis; infection with other species of mycobacteria may cause cross-reactions. The larger the reaction with a given antigenic dose, the greater is the probability that the reaction is specific for that antigen.

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